The remains of the Castle can be visited
Monmouth Castle

The ruins of Monmouth Castle, a Norman structure, stand in the town of Monmouth, Wales.
History+
The origins of Monmouth Castle date back to between 1066 and 1069, when William FitzOsbern initiated its construction. Around 1150, the complex underwent its first expansion. A change of ownership occurred in 1267, when the castle passed from Simon de Montfort to Edmund Crouchback, who also had it expanded. Later, Henry of Grosmont, the first Duke of Lancaster, became lord of the castle. By the end of the 13th century, the castle and town were fortified with a town wall.
Source: Wikipedia · Licence: CC BY-SA 4.0
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 51.8127, -2.7164